The present invention relates to an information recording and reproducing apparatus which comprises a combination of a plurality of disk drives for recording and reproducing data on and from disc-like recording media, and more particularly to such an apparatus which effects the recording and reproduction of data by using optical disks.
A conventional information recording and reproducing apparatus using a magnetic disk drive is disclosed in "NIKKEI ELECTRONICS 1985.3.11" (Mar. 11, 1985), pp. 159-187, as illustrated in FIG. 1 showing an arrangement of the conventional information recording and reproducing apparatus. In FIG. 1, illustrated at numeral 1 is a central processing unit (CPU) of a host computer which acts as a host device for recording and reproduction of data in and from a magnetic disk drive 12. Also included in the conventional information recording and reproducing apparatus are a main memory 2 which stores data and programs for the CPU 1 and a channel 3 for the CPU 1 which is for control of the input and output of data. Numeral 10 represents a disk control unit for controlling the magnetic disk drive 12 and a disk cache memory 11 to allow increased speed of data recording and reproduction processes. The disk cache memory 11 is a memory for increasing the speed of data access between the magnetic disk drive 12 and the CPU 1.
Operation of the conventional recording and reproducing apparatus with the above-mentioned arrangement will be described hereinbelow with reference to a data read and write flow chart illustrated in FIG. 2.
In a data-read operation, when access is first made with respect to one specific file, the disk control unit 10 simultaneously prefetches the next block of the file if it is absent in the cache memory 11. In this state, in response to a data-read request a from the CPU 1 with respect to one file, the disk control unit 10 refers to a directory DIR of the disk cache memory 11, which is placed in the disk control unit 10, to check whether or not the target data block of the pertinent file is present in the disk cache memory 11. In the case of the presence (i.e., hit) of the data block in the disk cache memory 11, the data block is transmitted from the disk cache memory 11 through the channel 3 to the main memory 2. In the case of the absence (miss) of the data block therein, several blocks (made up of sectors) including the target block are read out to the disk cache memory 11 and, of these blocks, the target block is transmitted to the main memory 2. Thus, if the disk cache memory 11 has an excellent hit probability, the data are directly transmitted from the disk cache memory 11 thereby sharply improving the data access speed.
In a data-write operation, well-known write-through system and write-back system are used which depend on the rewriting order between the disk cache memory 11 and the magnetic disk drive 12.
According to the write-through system, data are first written in the magnetic disk drive 12 (e). If the block corresponding to the recorded data is present in the disk cache memory 11, that is, in the case of hit, the corresponding block in the disk cache memory 11 is invalidated or the same data are also written in the disk cache memory 11 (f).
On the other hand, in the write-back system, data are preferentially written into the disk cache memory 11 (g), and when an unoccupied area for writing the data is absent in the disk cache memory 11 or when the block is not required or when the system determines that the data which have not been copied should be transmitted to the magnetic disk drive 12, data in the disk cache memory 11 are transmitted to the magnetic disk drive 12 where the data are recorded (h).
However, the above-described systems have both merits and demerits. In the write-through system, although data are not lost in response to a loss of power, a limitation is imposed on the write cyclability (the number of times of recording repetition) in the case of using a rewritable optical disk instead of the magnetic disk drive 12. That is, the number of times of writing data in response to requests of the host device directly affects the number of times of rewriting data in the optical disk. In an optical disk using a magneto-optical recording medium or a phase change recording medium which is arranged for effecting writing and erasure of a signal by the heating action due to a laser light beam, there is a problem in that the number of times of recording and erasure of data is limited due to the thermal stress of the medium caused by the light beam. For example, it is known that the write cyclability of a magneto-optical disk is about 1,000,000 to 10,000,000 and the write cyclability of the phase change recording medium for effecting the phase change in the amorphization state and crystallization state by using heat due to the laser light beam is about 1,000,000.
On the other hand, in the write-back system, in response to power-off prior to writing the data from the disk cache memory 11 into the magnetic disk drive 12, the data are lost from the disk cache memory 11, and therefore there is a problem that the disk cache memory 11 requires a battery back-up system. In addition, it is required to return the contents of the disk cache memory 11 to the magnetic disk drive 12 before cutting-off of the power source.